LCD devices have the advantages of achieving a lower power dissipation and a higher resolution, and thus are used in a variety of applications from a small-size portable telephone to a large-size television (TV) monitor. However, the contrast ratio of a LCD device alone is as low as 1000:1 at most, which is far lower than the contrast ratio of the other display devices such as a CRT, a plasma display device which is also used as a TV monitor, and a discharge-type display device referred to as filed-emission display (FED) device or surface-emission display (SED) device which has a contrast ratio of around tens of thousands. Thus, it has been pointed out that the LCD device has the problem of poor image-expression capability upon display of an image having a dark picturesque portion such as an image included in a movie.
For solving the above problem, a technique has been developed wherein the light intensity of a backlight source of the LCD device is controlled depending on the image to be displayed, without improving the contrast ratio of the LCD device itself, to thereby improve the contrast ratio of the LCD device as a whole. However, a cold cathode tube is generally employed in a surface-emitting backlight source, the cold cathode tube having a narrow dynamic range of the luminance. Thus, even if the light intensity of the backlight source is controlled depending on the image to be displayed, the improvement of the contrast ratio remains on the order of 2000:1 to 3000:1 at most. In addition, the cold cathode tube, which has an elongate shape, may suffer from a poorly controlled property of the luminance (brightness) in a partial screen area if there are a bright area and a dark area at the same time on the screen. This degrades the improvement of the contrast ratio obtained by the luminance control of the backlight source. In short, the LCD device suffers from reduction in the effective contrast ratio upon display of a lower-luminance area with a superior reproducibility on the screen, which also includes therein a higher-luminance area.
For avoiding the above problem, it is desired to markedly raise the contrast ratio of the LCD device. However, the contrast ratio of the LCD device alone is around 1000:1 at most, as described before. Patent Publications 1 and 2 describe a technique for improving the contrast ratio of the LCD device as a whole without significantly improving the contrast ratio of the LCD device itself. In this technique, a plurality of LC panels are stacked one on another to form a stacked LCD unit, thereby reducing the black luminance and thus improving the contrast ratio of the LCD unit as a whole. The term “black luminance” as used herein means a luminance on a screen supplied with a gray-scale level signal of a lowest luminance, i.e., black signal.
FIG. 6 shows a typical LCD unit including two LC panels stacked one on another. The LCD unit includes a first polarizing film 901, a first LC panel 941, a second polarizing film 902, a second LC panel 942, and a third polarizing film 903, which are consecutively arranged from an optical incidence side of the LCD unit. The first LC panel 941 includes a TN-mode liquid crystal (LC) layer 931, and a pair of transparent substrates 911 and 912 including transparent electrodes 921 and 922 in the vicinity of the LC layer 931. The second LC panel 942 includes a TN-mode LC layer 932, and a pair of transparent substrates 913 and 914 including transparent electrodes 923 and 924 in the vicinity of the LC layer 932. The transparent electrodes 921 and 923 of the LC panels 941 and 942 are pixel electrodes to which a drive signal is supplied from a drive circuit 951, whereas the transparent electrodes 922 and 924 of the LC panels 941 and 942 are common electrodes.
If the contrast ratio of the LC panel 941, 942 alone is measured using a laser beam, the contrast ratio will be around 10 to 15. On the other hand, if the overall contrast ratio of the LCD unit including the LC panels 941, 942 is measured, the overall contrast ratio improves up to around 100:1. If the contrast ratio of a LCD unit including three LC panels is measured, the contrast ratio will improve up to around 1000:1, whereby a superior contrast ratio far exceeding the limit of the contrast ratio of the LC panel alone will be achieved. The Patent Publications as described above are as follows:    Patent Publication 1—JP-1989-10223A    Patent Publication 2—JP-UM-1984-189625A
In the LCD unit described in Patent Publication 1, two LC panels 941, 942 stacked one on another are used to achieve a higher contrast ratio, wherein the two LC panels are driven by a common signal supplied from a single signal source. In this configuration, corresponding positions (pixels) of the two LC panels overlapping each other are driven by the same signal at the same timing irrespective of whether the picture to be displayed therein is a still picture or a moving picture. This may cause overlapping of the transmittance change of both the LC panels in a frame period from the start of writing the image data to the end of holding the image data, thereby amplifying the amount of transmittance change or flicker in the frame period. The amplified flicker incurs the problem of sense of discomfort in an observer of the LCD unit.